I had no idea there was magma beneath Socorro, New Mexico. When I read about it in this month’s Geology, my first reaction was OMG WE’RE GONNA DIE!. (I’ve been occasionally using the electron microprobe at New Mexico Tech to look at rocks that were metamorphosed around a 380-million-year-old granite. I had no idea that the same kinds of processes were going on, right then, beneath my feet.)

The magma body is 19 km deep in the crust. That’s about 2/3 of the way to the mantle – pretty far from the surface. But the effects are still noticeable, at least if you look at interferometric synthetic aperture radar (which uses satellites to figure out how much the ground surface has changed elevation). The elevation changes are around 2.5 mm/year, which means that I can be excused for not noticing them while I was on the probe.

(Below the fold: a paradox, and the reason why this research was published in Geology.)

Geologists (except for me) have known about the magma body for some time, based on evidence from both geophysics and from the landscape, but there’s a paradox. The original landscape evidence came from river gradients: the Rio Grande is gentler upstream of Socorro and steeper downstream, as if the river had been warped upward.(There’s also an older river deposit south of Socorro that is steeper than the current river gradient, so much that it might mean that Socorro has been uplifted 85 meters). But those observations are difficult to reconcile with the thickness of the magma body. It’s only 150 meters thick (although it’s 42 km in diameter), which means it should only take a few hundred years to freeze solid. But landscape looks like it’s been warping for thousands of years. That’s an order of magnitude difference. Ouch. So what’s going on?

Noah Finnegan and Matthew Pritchard (Geology, March 2009) found a solution to the problem – and in the process, give a nice example of ways that geologists can test hypotheses about the past. To figure out how long the uplift has been going on, they looked at a number of different river and alluvial fan surfaces that cross the magma body. Not just the Rio Grande, but also the Rio Puerco and Rio Salado, which join the Rio Grande above Socorro, and old terrace surfaces between Albuquerque and Socorro. (An alternate explanation for the Rio Grande’s shape is that its gradient changes because of the amount of sediment delivered to it by the Rio Puerco. If the magma is responsible, all the rivers and terraces should be warped in ways controlled by the behavior of rock above the magma body. If the sediment is responsible, however, the Rio Grande should behave differently from its tributaries.)

The Rio Puerco and the Rio Salado show no sign of longterm uplift over the magma body. Neither do the old alluvial fan surfaces. The evidence for uplift during the past century, however, looks good. That means that the magma is young. Really young.

So should I be worried? Well, the mid-crust is pretty far away. On the other hand, in places like the Rio Grande Rift (where the continental crust is being stretched), it’s possible that hot, dense basaltic magma pools in the lower crust, but causes the rock around it to melt. That can lead to eruptions of viscous rhyolite magma. (Think supervolcanoes. Big calderas rather than mountains.) It’s happened in New Mexico in the not-too-distant past (geologically speaking, that is). I don’t know what type of magma is sitting under Socorro, or what type of rock surrounds it. (There are 1.6-billion-year-ish metamorphosed rocks at the surface nearby, but I don’t know what the deeper part of the crust looks like. Metamorphosed mud is easy to melt; the basaltic roots of a Precambrian volcano would take higher temperatures. Either one could potentially be down there.) My gut feeling is that it’s not worth worrying about.

Thanks for pointing this out. This’ll make a fun newspaper story (when I can carve out the time). I wrote about the magma body once before, the source of one of my great trivial newspaper triumphs: getting the cliche “the size of Rhode Island” into the newspaper.

And don’t you have some Quaternary or a bit older andesite/basaltic volcanics in the Socorro area and up and down the rift? I suppose they are associated with the structural setting/faulting of the Rio Grande rift, so how deep were they sourced?

And yes, there are young volcanics up and down the Rio Grande Rift. There are some in road cuts just south of Albuquerque, for instance. I’ve assumed they are basalt (though I’ve never stopped to look at them – it’s the Interstate, after all, and I’ve always been trying to go someplace else), but I don’t know. Presumably their ultimate source is the mantle. But I don’t know what allowed them to reach the surface, while the Socorro magma body has spread out down at 19 km depth.